1. Field of the Invention
The invention relates to a foil seal lamp in which in the seal area a metal foil of molybdenum and an outer lead of molybdenum are installed.
2. Description of Related Art
A conventionally known foil seal lamp is described below. FIG. 1 shows a foil seal lamp of the filament lamp type with bilateral seal areas. FIG. 2 shows the seal area of the foil seal lamp as shown in FIG. 1 in an enlargement.
In the figure, a filament lamp 10 has seal areas 3 which are formed on the two ends of a glass bulb 1, and in each of which a metal foil 2 of molybdenum is installed. An outer lead 4 of molybdenum is connected to one end of the respective metal foil 2 by welding and projects from the outer end face 3A of the respective seal area 3 to the outside. On the other hand, within the bulb 1 there is a filament 5. The two ends of the filament 5 are welded via inner leads 6 to the metal foils 2 which are located on the two ends.
In the seal area 3 of such a filament lamp 10, as is shown enlarged in FIG. 2, in the vicinity of the outer lead 4, there is an extremely small cavity G which extends from the outer end face 3A of the seal area 3 to the metal foil 2.
The reason for forming this cavity G is described below. In the formation of the seal area by a pinch seal, major tensile stress does not arise in the glass because the metal foil is relatively thin. In this way, the metal foil is located directly tightly adjoining the glass. A relatively large outer lead, however, is not entirely located directly tightly adjoining the glass because, as a result of the high viscosity of the glass, the glass does not flow adequately according to the shape of the outer lead, and because the difference between the coefficients of thermal expansion between the outer lead and the glass is large. Thus, the cavity G is formed. Therefore, it is impossible, in fact, to completely eliminate the cavity G.
In such a foil seal lamp, during operation, the temperature of the seal area rises. If the metal foils and the outer leads reach a temperature of at least 350° C., rapid oxidation of the metal foils and the outer leads by the air which has penetrated into the cavity G progresses, by which MoO3 is formed on the surfaces of the metal foils and the outer leads. In this way, the volume of the metal foils and the outer leads which are installed in the seal areas expands. This leads to crack formation in the seal areas. Therefore, this has resulted in cases in which the lamp was ultimately destroyed.
A lamp with a pinch seal arrangement was described above. In a discharge lamp with a vacuum shrink seal arrangement, a cavity is formed between the metal foil of molybdenum and the outer lead of molybdenum which are likewise installed in the seal area, and the glass. Therefore, the above described disadvantage has also occurred here.
Various techniques have been developed to suppress this oxidation of the metal foils of molybdenum and the outer leads of molybdenum.
Previously, U.S. Pat. No. 3,420,944 disclosed coating half of the side of the seal area end of the metal foil of molybdenum with thin chromium. In this way, the disadvantage of oxidation was eliminated to a certain extent. However, there was the disadvantage of a reduction of the strength of adhesion to the glass.
In U.S. Pat. No. 3,793,615, a new attempt was made. Here, the chromed layer was made conical or tapered. The chromium layer on the outer end in the vicinity of the weld spot of the outer lead is relatively thick, while in a metal foil which forms the hermetically sealed area it is thin. Thus, the glass-adhesive property was improved. This arrangement somewhat suppressed the oxidation of the metal foil of molybdenum.
U.S. Pat. No. 5,021,711 discloses an ion implantation process in which the metal foil is coated with Al, Cr—Al, SiC or Si3N4, besides Cr. Published German Patent Application DE 30 06 846 discloses a process in which, by sputtering, CVD, ion implantation or the like, the metal foil is coated with Ta, Nb, La, Sc, Hf or the like, besides Cr. In this process, there is the disadvantage of high production costs because coating is done beforehand, prior to sealing.
In such a process in which the metal foil and the outer lead are coated with chromium, there are the disadvantages of complicated production processes and high production costs because sealing takes place after coating of the metal foil and the outer lead with chromium.
Furthermore, recently, environmental concerns have become more critical. There is the tendency for material with high environmental burden to be limited, chromium not constituting an exception either.
The possibility that metallic chromium will change to hexavalent chromium is low. The mist which is formed by the electrolytic bath in the galvanization process and which contains hexavalent chromium is said, however, to cause lung cancer. This means that the environmental effect in the production process is regarded as a problem. Furthermore, a chromium coating which decomposes in chromous acid and chromium chloride, as was described in U.S. Pat. No. 3,420,944, due to production of hexavalent chromium is inherently environmentally harmful and therefore disadvantageous.
Japanese patent disclosure document HEI 9-12335 discloses a process in which the cavity formed in the seal area is sealed with molten sealing glass which contains 1 to 18% Tl2O3 besides 55% to 85% Sb2O3 and 5% to 30% B2O3. But since Tl2O3 is used, the effect on the environment is considered disadvantageous.
A process in which, after sealing, a sealant in the form of an aqueous solution is added to the cavity is disclosed in U.S. Pat. No. 4,918,353. In this process, an alkali metal acid salt is added to the cavity.
Japanese patent specification HEI 6-54657 also discloses a process in which, after sealing, a sealant with lead or lead oxide as the main component is produced in the cavity. However, since lead oxide is used, the effect on the environment is regarded as more and more disadvantageous.
In these processes a sealant is formed after sealing so that there is, therefore, the advantage that, compared to U.S. Pat. Nos. 3,420,944; 3,793,615; 5,021,711, and German patent 30 06 846, the production costs are relatively low.
Even when these tests were carried out, in an environment with a temperature of the seal area of greater than 400° C., oxidation of the metal foil of molybdenum and of the outer lead of molybdenum which are exposed to the cavity of the seal area could not be reliably prevented. Furthermore, there was the disadvantage that to prevent oxidation a substance must be used which has an adverse effect on the environment.